British light heavyweight Buatsi made a winning yet unimpressive debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles never quite clicked, with excessive holding disrupting the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest display but what defines champions? They win," the victor stated in the ring as he was faced boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts felt the challenger, thirty-one, merited the win but the British boxer suffered the second defeat of his 28-fight career.
Buatsi Earns Win in Slow Match
The fighter's advancement has slowed in recent years through promotional disputes and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who utilized a solid defense to narrowly win the better periods.
The crowd's patience decreased during constant clinching, with Parker going down multiple occasions during grappling.
The underdog closed well, but many rounds were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester contest to win the continental championship.
The venue fell silent for a deeply poignant tribute prior to the bout as a bell ceremony tolled in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the quiet gave way to a chant of "only one Ricky Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was floored by a powerful punch flush on the face in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was injured soon, significant bruising developing under his eye as the pair traded powerful shots.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the action rarely dropped over the full distance.
The victor edged the result with scoring tallies of even score, narrow lead and wider gap, but both fighters were relentless and courageous, everything their hero the legend represented.
